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A SHORT HISTORY of AIR INTERCEPT RADAR and the BRITISH NIGHT-FIGHTER, PART ONE 1936 – 1945, by Ian White

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A SHORT HISTORY of AIR INTERCEPT RADAR and the BRITISH NIGHT-FIGHTER, PART ONE 1936 – 1945, by Ian White A SHORT HISTORY OF AIR INTERCEPT RADAR AND THE BRITISH NIGHT-FIGHTER, PART ONE 1936 – 1945, by Ian White. Following the pioneering work undertaken by Robert (later Sir Robert) Watson-Watt, Arnold Wilkins and A.P.Rowe at Daventry and later at the Bawdsey Manor experimental station in Suffolk, in the design of the early warning (EW) radar system code-named Chain Home (CH), the Air Staff’s Member for Research & Development, Air Marshal Sir Hugh Dowding, in conjunction with the government scientist, Professor Henry (later Sir Henry) Tizard, turned their attention towards the design and development of an airborne radar set. 1.5 METRE AI RADAR During August 1936 Watson-Watt, by then the Superintendent of the Air Ministry Research Establishment (AMRE) at Bawdsey, formed an Airborne Group under the leadership of Dr Edward ‘Taffy’ Bowen to manage the development of air intercept (AI) radar. The first trials of a complete airborne radar system (transmitter, receiver and display) installed in a Heyford bomber and operating on 6.7 metres (45 MHz), were conducted by Bowen in March 1937. By August of that year, the wavelength, and hence the size of the aerial system, had been reduced to 1.25 metres and installed in Avro Anson, K6260, for flight trials at Martlesham Heath. This set, which demonstrated ranges of 2 - 3 miles (3.2 - 4.8 km) in the ‘sea search’, or air-to-surface vessel (ASV), mode and a mile (1.6 km) in AI, formed the basis for all the wartime airborne radars. With the wavelength increased to 1.5 metres (200 MHz) to improve the sensitivity, the first hand built examples of AI Mk.I were installed in six Blenheim IVf fighters of a special flight of No.25 Squadron at Northolt in August 1939, whilst similar examples of ASV Mk.I were fitted to a very small number of Coastal Command aircraft. The first operational AI, AI Mk.III, (Mk.II was a failure) were installed in the Blenheim Ifs of Nos.23, 25, 29, 219, 600 & 604 Squadrons in May 1940. The first AI success of the War occurred on the night of July 22/23rd 1940, when a Blenheim If of the Fighter Interception Unit (FIU), piloted by F/O Glyn ‘Jumbo’ Ashfield, with Sgt Reginald Leyland operating the radar and P/O Morris observing, intercepted and shot down, what is believed to have been a Dornier Do 17Z off Bognor Regis, Sussex. The Blenheim/AI Mk.III combination’s principal disadvantage lay in the modest performance of the aircraft and the poor minimum range of the radar. In an attempt to overcome the former, Fighter Command’s C-in-C, Air Chief Marshal Sir Hugh Dowding, directed that future night-fighters were to be based on the Bristol Beaufighter, which had both the performance and the armament required of a night-fighter. To match this AMRE called upon the assistance of the EMI Company, and particularly the services of its chief designer, Alan Blumlein, to produce the first practical, series production AI radar, AI Mk.IV. It should be noted that the turning of the aerial system from the horizontal to the vertical plane (polarisation) and the incorporation of a central modular to control the system, brought about a significant improvement in performance of the minimum range over that of AI Mk.III. (the Author) Bristol Blenheim If fitted with the aerial system of AI Mk.III. It should be noted that this aircraft has had the gunner’s turret removed to save some 800-lbs of weight and improve its performance (slightly). It was with the Beaufighter/AI.IV combination that Fighter Command began the Winter Blitz of 1940/41, albeit in limited numbers. The Beaufighter entered service in September 1940 and scored its first victory on the night of November 15/16th, when an aircraft of No.604 Squadron destroyed a Junkers Ju 88A-5 near Chichester, Sussex. Overall, Fighter Command’s performance in the autumn and the early winter of 1940 was poor, due in part to the limited number of Beaufighters, a lack of trained aircrews, poor radar serviceability and the defence’s inability to track enemy aircraft once they had passed through the CH screen. This last problem had been foreseen by the staff at AMRE, who undertook interception trials earlier in the year, which, in turn, led to the development of a ground control of interception (GCI) radar, based on elements of the 1.5 metre Chain Home Low (CHL) radar, that was designed to fill the low level gaps in the CH chain, and the Army’s gun-laying (GL) radar. With its rotating aerial and plan position indicator (PPI) display, the first mobile GCI (AMES Type 8) was installed at Durrington in November 1940 and successfully tested under operational conditions the following month. By April 1941, six Type 8s were in operational service. An improved version, AMES Type 7, that was capable of handling multiple interceptions, was deployed at permanent GCI stations from the summer of 1942. (British Aerospace) Bristol Beaufighter If fitted with the aerial system of AI Mk.IV. The arrowhead folded dipole transmitter element is mounted on the nose, with the port and starboard azimuth (directional) dipoles on the wing outboard sections and the elevation (up/down) unipole elements above and below the starboard wing tip. This Beaufighter type was flown by both 600 & 604 Squadrons from September 1940 to April 1943. With the help of the GCI chain and improvements in the delivery of Beaufighters and radar sets, combined with the availability of better trained night-fighter crews from the Operational Training Unit (OTU) organisation and radar mechanics from the Radio Schools, saw the number of enemy aircraft destroyed begin to rise steadily during the late winter and spring of 1941 - three in January, four in February, twenty-two in March, forty-eight in March and ninety-six in May. It should be noted, however, that these numbers represented the combined figures for radar-equipped fighters and cats- eye’s day-fighters (Hurricanes and Defiants) operating in the night role. At this juncture (spring 1941) in the development of Britain’s night defences, Fighter Command’s squadrons were equipped predominantly with Beaufighters (AI Mk.IV), supported by a few Blenheim If/IVf (AI Mk.III) squadrons and one flying the night-fighter version of the Douglas Boston bomber, the Havoc (AI Mk.IV). (Crown Copyright) The Radar Operator’s (R/O) Display Unit for AI Mk.IV, with the elevation (up/down) CRT on the left and the azimuth (left/right) CRT on the right. Range was shown on both tubes. (The Author) The display shows a target aircraft at a range of approx 3 miles, above and to port of the fighter. Variations in the quality of radar operators (R/O) and with it the need for the pilot to control the latter parts of an interception, led to the development of AI Mk.V and the introduction of the pilot’s indicator (a small cathode ray tube [CRT] that might be regarded as the precursor of the modern head-up display - HUD) in the cockpit. Installed from April 1942 onwards in the superb de Havilland Mosquito, AI.V claimed its first victim on the night of 24/25th June, when a Mosquito NF.II from No.151 Squadron destroyed a Dornier Do 217E-4 over the North Sea. The final metric AI radar, AI Mk.VI, a fully automatic system for single-seat fighters, was developed and test flown in 1942. However, with the removal of single-seat night-fighters from Fighter Command’s inventory in 1942 and the imminent introduction of centimetric radar technology, AI.VI production was cancelled and the sets were converted to Monica tail-warning radars for Bomber Command. (Crown Copyright via IWM) Beaufighter IIf powered by two R-R Merlin 20-series engines and fitted with AI Mk.IV radar. A number of these aircraft were employed operationally by 600 Squadron from April 1941 to April 1942. CENTIMETRIC RADAR The promise of shorter ‘centimetric’ wavelengths and narrower beams devoid of ground returns, became a reality in February 1940, when J.T.Randle and H.A.H.Boot ran the first resonant cavity magnetron valve at Birmingham University. This device, which some have rightly described as one of the most significant technological developments of World War Two, demonstrated a test-bench pulsed output of 10 kW on a wavelength of 9.8 cms (3,060 MHz). With the support of the GEC Company, the magnetron was gradually developed to the point where a small number were made available to the scientists at AMRE in May 1940, which by then had moved to Worth Matravers, near Swanage. In great secrecy, a team, initially under the leadership of Dr Herbert Skinner, was formed to design an AI radar around the magnetron valve, whilst Dr Bernard Lovell examined the feasibility of building a parabolic dish aerial and installing it in a radio transparent (perspex) radome fitted to a Blenheim IV. (Crown Copyright) The de Havilland Mosquito NF.II prototype, W4052, showing the aerial system of AI Mks.IV & V (they were identical) and the all-black sooty night finish favoured by Fighter Command in the early part of the war. The R/O’s Indicating Unit display for AIS and AI Mks.VII & VIIIA comprised a single screen. The spiral scanning system employed on these radars produced an arc to indicate a target and the ground returns were shown as horizontal lines, which also, conveniently, provided an artificial horizon.
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